Sandwiches of multiple, thin, controlled-thickness layers of transparent or semi-transparent materials, where the layers are of materials having two or more different indices of refraction, can be used to form interference filters, dichroic mirrors, and other optical components capable of interacting with electromagnetic radiation of particular wavelengths. For simplicity, such devices are referred to herein as multilayer optical components, and operate through interference effects that occur because there is at least some reflection at each boundary between layers of the sandwich. The reflected and transmitted light from each of the boundaries may sum or cancel at particular wavelengths. For example, light reflections from two boundaries can cancel each other if the round-trip distance between boundaries is N wavelengths plus half a wavelength, or can reinforce each other if the round-trip distance between boundaries is N+1 wavelength (for integer N greater than or equal to zero).
The amount of reflection at each boundary depends in part on a difference between the indices of refraction of adjacent layers forming the boundary. The optical properties of the sandwich, and thus of the multilayer optical component, are determined in part by the materials of each layer, as well as their thicknesses.
By careful selection and application of layers of particular thickness, layer order, and index of refraction, multilayer optical components functional as optical bandpass filters, optical band-stop filters, wavelength-selective mirrors, antireflection coatings, and other optical devices are commonly made. U.S. Pat. No. 3,247,392 to Thelen (1961) describes some of the optical components available of this type. While these interference effects can be observed in liquids as well as solids, and materials of biological as well as mineral or manufactured origin, most manufactured devices rely on layers of solid materials. These multilayer optical components are capable of spectrally modifying incident electromagnetic radiation in a manner not reliant on selective absorption, and a manner dependent on interference effects, because their light reflection and/or transmission is wavelength-dependent such that they may separate, or transmit or reflect differently, two components of incident light having different wavelengths.
It is occasionally desirable to provide tunability of optical components. For example, a strong demand exists for tunable lasers. Some multilayer optical components have been built that are tuned to desired wavelengths by altering the angle of incident light arriving on the component's surface; since light arriving at an angle from normal passes through more material between boundaries than light arriving normal to the surface, the light arriving at an angle exhibits interference effects at longer wavelengths than the light arriving normal to the surface. Altering the angle of incident light is typically performed by rotating components. It is not always convenient to optically tune components by rotating them.
There are existing methods, based on those used for semiconductor fabrication, for creating small structures that have a top plate suspended over an air-filled space above a substrate or underlying layer. As an example, U.S. Pat. No. 5,526,951, the disclosure of which is incorporated herein by reference, describes a process for forming micromirrors, each of which are supported by a pivoting support structure and suspended over a narrow air space or cavity above an underlying layer or substrate.